Membrane-electrode assemblies for electrochemical cells

ABSTRACT

A combination, unitary, membrane and electrode assembly with a solid polymer electrolyte membrane, and first and second electrodes at least partially embedded in opposed surfaces of the membrane. The electrodes each comprise a respective group of finely divided carbon particles, very finely divided catalytic particles supported on internal and external surfaces of the carbon particles and a proton conductive material intermingled with the catalytic and carbon particles. A first group of finely divided carbon particles forming the first electrode has greater water attraction and retention properties, and is more hydrophilic than a second group of carbon particles forming the second electrode. In a preferred method, the membrane electrode assembly of the invention is prepared by forming a slurry of proton conductive material and at least one group of the carbon and catalyst particles. The slurry is applied to the opposed surfaces of the membrane and heated while being pressed to the membrane for a time and at a temperature and compressive load sufficient to embed at least a portion of the particles into the membrane.

FIELD OF THE INVENTION

This invention relates to membrane and electrode assemblies for use withelectrochemical cells having solid polymer electrolyte membranes.

BACKGROUND OF THE INVENTION

Electrochemical cells are desirable for various applications whenoperated as fuel cells. During operation, fuel cells typically consumehydrogen and oxygen or air to provide electrical power. Although suchcells are used in space applications, they have not been favored fortransportation use. The cost of power produced by a fuel cell is severalorders of magnitude greater than the cost of other power generationalternatives, partly because of the poor utilization of precious metalcatalysts in conventional electrodes. However, power produced fromhydrogen is desirable because hydrogen is environmentally acceptable andhydrogen fuel cells are efficient. Important features of a fuel cellinclude reaction surfaces, catalysts, ion conductive media, and masstransport media. Such features are typically included in membrane andelectrode assemblies of a fuel cell. Therefore, it is desirable toimprove the catalyst utilization in such assemblies to render fuel cellsmore attractive for transportation use.

SUMMARY OF THE INVENTION

There is provided a combination, unitary, membrane electrolyte andelectrode assembly with a solid polymer electrolyte membrane havingfirst and second opposed surfaces and a thickness (intermediate region)therebetween. First and second electrodes are each adhered to arespective one of the first and second membrane surfaces. Each of theelectrodes is well adhered to the membrane and preferably at leastpartially embedded in the membrane.

The electrodes each comprise a respective group of finely divided carbonparticles, very finely divided catalytic particles supported on internaland external surfaces of the carbon particles and a proton conductivematerial intermingled with the catalytic and carbon particles.

A first group of finely divided carbon particles forming the firstelectrode has greater water attraction and water retention propertiesand is relatively more hydrophilic and less hydrophobic than a secondgroup of carbon particles forming the second electrode.

Preferably, each one of the first and second groups of the finelydivided carbon particles is characterized by a pH, the first group has apH in the range of about 6 to about 7, and the second group a pH ofabout 8 to about 10. Thus, the first group is relatively more acidic andcorrespondingly less basic than the second group.

The carbon particles define pores some of which are internal pores inthe form of holes in the carbon particles; other pores are gaps betweenadjacent carbon particles.

Preferably, the first group of carbon particles has an average pore size(radius) in the range of about 90 to about 110 angstroms and the averagepore size of the second group is in the range of about 60 to 80angstroms. Conveniently, the average pore size of the first group isachieved by ball-milling which produces a suitable distribution of poresizes and relatively random distribution of equivalent particlespherical diameters.

The preferred membrane electrolyte material and ion (proton) conductivematerial are each perfluorocarbon sulfonic acid polymer. Optionally, theion conductive material also includes polytetrafluoroethylene.

In another embodiment, the first and second electrodes further compriserespective first and second Teflonated (polytetrafluoroethylene coated,impregnated) graphite sheets, adhered to a respective surface of themembrane. The first group of particles is disposed between the firstsurface of the membrane and the first sheet; the second group isdisposed between the second surface and the second sheet.

In a preferred method, the membrane electrode assembly of the inventionis prepared by first forming a slurry of proton conductive material andat least one group of finely divided carbon particles and very finelydivided catalytic particles supported on and in the carbon particles.Next, the slurry is applied to first and second surfaces of themembrane.

Then the applied slurry is heated while being pressed to the membranefor a time and at a temperature and compressive load sufficient tosoften the membrane and at least partially embed at least a portion ofthe particles in the membrane to thereby form the first and secondelectrodes. The embedded or inset particles are at least partially setin surfaces of the membrane although they may not be totally encompassedby the membrane or disposed below its surface.

In another embodiment, the slurry is applied to respective first andsecond Teflonated graphite sheets. Then, the wetted side of the firstsheet is applied to the first surface of the membrane and the wettedside of the second sheet is applied to the second surface of themembrane. The applied sheets are hot-pressed to the membrane while beingheated for a time and at a temperature and compressive load sufficientto soften the membrane and at least partially embed at least a portionof the particles in the membrane to thereby form the first and secondelectrodes.

Desirably, before use, and if needed the method further includestreatment to remove any organic and inorganic impurities and to convertany non-hydrogen cation exchange sites of the proton conductive materialto the hydrogen form.

Desirably, before the step of heating while pressing, the applied slurryis dried by heating for a time and at a temperature sufficient tovaporize any volatile liquid components of the slurry

Preferably, the step of heating while pressing is conducted at about1,000 to about 2,000 pounds per square inch (lb/in²) compressive loadfor about 1 to about 5 minutes at a temperature of about 120° C. toabout 150° C. The compressive load may vary with time. That is, lessload and longer times may be used, and the converse also applies

Preferably, the proton conductive material and the catalytic and carbonparticles forming each electrode are in a proportion based on 100 parts,of 30 to about 50 parts proton conductive material and the balance beingcatalytic and carbon particles. And, the platinum and carbon particlesare in a proportion based on 100 parts by weight of up to about 15 partsplatinum and the balance being carbon particles.

The embedding of electrodes into the membrane under pressure, providesfor a continuous path of proton conductive material from one side of themembrane electrode assembly to the other. The intimate intermingling ofproton conductive material with catalyst and carbon particles provides acontinuous path for protons to the catalyst site where reaction occurs.The method also achieves a relative peak distribution of catalyticparticles adjacent the membrane at each electrode.

The membrane electrode assembly of the invention advantageously producesrequired power output at a very low catalyst loading of less than about0.07 milligrams per cm² of electrode surface area. Further, good resultsare possible with the Pt loading of one electrode being less than thatof the other, so that the total cell catalyst loading may be less thanabout 0.13 milligrams of the platinum particles per cm² of the surfacearea of either one of the electrodes.

Thus, the new membrane electrode assembly of the invention providesrelatively high power output with unexpectedly low catalyst loading.

These and other objects, features and advantages will become apparentfrom the following description of the preferred embodiments, appendedclaims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an unassembled electrochemical fuel cellhaving a membrane electrode assembly according to the invention.

FIG. 2 is a pictorial illustration of a cross-section of a membraneelectrode assembly according to the invention.

FIG. 2(a) is a pictorial illustration of a membrane as in FIG. 2 andhaving graphite sheets.

FIG. 3 is a pictorial illustration showing a magnified view of a portionof FIG. 2.

FIG. 4 is a pictorial illustration showing a magnified view of a portionof FIG. 3.

FIG. 5 is a graph of hydrogen-oxygen fuel cell performance utilizingvarious carbon particles as cathode electrode substrates.

FIG. 6 is a graph of hydrogen-oxygen fuel cell current density versus pHof various carbon particles utilized as cathode electrode substrates.

FIG. 7 is a multiple regression plot of current density showing thecorrelation between current density and pH and mesopore area for variouscarbon particle substrates.

FIGS. 8 and 9 are graphs of hydrogen-oxygen fuel cell performance formembrane electrode assemblies of the invention and for comparativeassemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electrochemical cell 10 with a combinationmembrane electrolyte and electrode assembly (MEA) 12 incorporatedtherein is shown in pictorial unassembled form. Electrochemical cell 10is constructed as a fuel cell. However, the invention described hereinis applicable to electrochemical cells generally. Electrochemical cell10 comprises stainless steel endplates 14,16, graphite blocks 18,20 withopenings 22,24 to facilitate gas distribution, gaskets 26,28, carboncloth current collectors 30,32 with respective connections 31,33 and themembrane electrolyte and electrode assembly (MEA) 12. The two sets ofgraphite blocks, gaskets, and current collectors, namely 18, 26, 30 and20, 28, 32 are each referred to as respective gas and current transportmeans 36,38. Anode connection 31 and cathode connection 33 are used tointerconnect with an external circuit which may include other fuelcells.

Electrochemical fuel cell 10 includes gaseous reactants, one of which isa fuel supplied from fuel source 37, and another is an oxidizer suppliedfrom source 39. The gases from sources 37,39 diffuse through respectivegas and current transport means 36 and 38 to opposite sides of the MEA12.

FIG. 2 shows a schematic view of the assembly 12 according to thepresent invention. Referring to FIG. 2, porous electrodes 40 form anode42 at the fuel side and cathode 44 at the oxygen side. Anode 42 isseparated from cathode 44 by a solid polymer electrolytic (SPE) membrane46. SPE membrane 46 provides for ion transport to facilitate reactionsin the fuel cell 10.

SPE Membrane

The solid polymer electrolyte (SPE) membrane 46, of the presentinvention is well known in the art as an ion conductive material.Typical SPE membranes are described in U.S. Pat. Nos. 4,272,353 and3,134,697, and in Journal of Power Sources, Volume 29 (1990), pages367-387.

The SPE membranes or sheets are ion exchange resin membranes. The resinsinclude ionic groups in their polymeric structure; one ionic componentof which is fixed or retained by the polymeric matrix and at least oneother ionic component being a mobile replaceable ion electrostaticallyassociated with the fixed component. The ability of the mobile ion to bereplaced under appropriate conditions with other ions imparts ionexchange characteristics to these materials.

The ion exchange resins can be prepared by polymerizing a mixture ofingredients, one of which contains an ionic constituent. One broad classof cation exchange, proton conductive resins is the so-called sulfonicacid cation exchange resin. In the sulfonic acid membranes, the cationion exchange groups are hydrated sulfonic acid radicals which areattached to the polymer backbone by sulfonation.

The formation of these ion exchange resins into membranes or sheets isalso well known in the art. The preferred type is perfluorinatedsulfonic acid polymer electrolyte in which the entire membrane structurehas ion exchange characteristics. These membranes are commerciallyavailable, and a typical example of a commercial sulfonatedperfluorocarbon, proton conductive membrane is sold by E.I. Dupont deNemours & Co. under the trade designation NAFION. Another is sold by DowChemical. Such proton conductive membranes may be characterized bymonomers of the structures: ##STR1##

In the electrochemical fuel cell 10 of the invention, the membrane 46 isa cation permeable, proton conductive membrane, having H⁺ ions as themobile ion; the fuel gas is hydrogen and the oxidant is oxygen or air.The overall cell reaction is the oxidation of hydrogen to water and therespective reactions at the anode 42 and cathode 44 are as follows:

    H.sub.2 =2H.sup.+ +2e

    1/2 O.sub.2 +2H.sup.+ +2e=H.sub.2 O

Since hydrogen is used as the fuel gas, the product of the overall cellreaction is water. Typically, the product water is rejected at thecathode 44 which is the electrode 40 on the oxygen side. Typically,water then escapes by simple flow or by evaporation. However, means maybe provided if desired, for collecting the water as it is formed andcarrying it away from the cell.

Water management in the cell is important and key to the successfullong-term operation of electrochemical fuel cell 10. Spatial variationsof water content within the polymeric electrolyte membrane 46 of acurrent-carrying fuel cell 10 result from the electroosmotic dragging ofwater with proton (H⁺) transport from anode to cathode, the productionof water by the oxygen reduction reaction at the cathode, humidificationconditions of the inlet gas stream, and "back-diffusion" of water fromcathode to anode.

The invention provides catalyst substrates comprising carbon, andcatalyst distribution and loadings tailored to the requirements of thehydrogen oxidation and oxygen reduction reactions occurring in the fuelcell 10. In addition, more effective proton transfer is provided byembedding electrodes 40 into the membrane 46. Accordingly, the MEA 12 ofcell 10 has membrane 46 with spaced apart first and second opposedsurfaces 50,52, a thickness or an intermediate membrane region 53between surfaces 50,52. Respective electrodes 40, namely anode 42 andcathode 44 are well adhered to membrane 46, at a corresponding one ofthe surfaces 50,52. The catalyzed carbon particles 60 are adhered to themembrane 46 and at least a portion of the particles 60 are at leastpartially embedded in membrane 46.

The embedded or inset particles are at least partially set in surfacesof the membrane although they may not be totally encompassed by themembrane or disposed below its surface.

As shown in FIG. 3, each of the electrodes 40 are formed of acorresponding group of finely divided carbon particles 60 and veryfinely divided catalytic particles 62 and a proton conductive material64 intermingled with the particles.

It should be noted that the carbon particles 60 forming the anode 42differ from the carbon particles 60 forming the cathode 44 ascharacterized by physical properties of such particles described below.The first group of finely divided carbon particles 60 forming the anode42 has greater water attraction and retention properties, and isrelatively more hydrophilic and less hydrophobic than the second groupforming the cathode 44. In addition, the catalyst loading at the anode42 is preferably less than the catalyst loading at the cathode 44.

Although the characteristics of the carbon particles and the catalystloading differ for anode 42 and cathode 44, the basic structure of thetwo electrodes 40 is otherwise generally similar, as shown in theenlarged portions of FIGS. 3 and 4, each taken from FIG. 2.

Referring now to FIG. 4, a further magnified view of a portion of aporous gas diffusion electrode 40 in accordance with the presentinvention is presented. Carbon particles 60 are provided to supportcatalyst particles 62, preferably of platinum (Pt), which are preferablysupported on internal and external surfaces of the carbon particles 60.Carbon particles 60 define pores 70 some of which are internal pores 70in the form of holes in the carbon particles; other pores 70 are gapsbetween adjacent carbon particles. The pores 70 enable gases topenetrate within the electrode 40 and in and around carbon particles 60and pores 70 for electrochemical reactions to occur adjacent catalyst62.

In order to provide a continuous path to conduct H⁺ ions to the catalyst62 for reaction, the proton (cation) conductive material 64 is dispersedthroughout each of the electrodes 40, is intermingled with the carbonand catalytic particles 60,62 and is disposed in a plurality of thepores 70 defined by the catalytic particles. Accordingly, in FIG. 4, itcan be seen that the proton conductive material 64 encompasses carbonand catalytic particles 60,62.

The characteristics of the carbon particles 60 are of key importance tooptimize reactions of the cell and satisfy the differing requirements ofthe anode 42 and cathode 44. Preferably, the first group of particles 60at the anode 42 is more acidic and correspondingly less basic than thesecond group at the cathode and the first group (anode electrode) has apH less than that of the second group (cathode electrode). (Refer toTables I and II and FIGS. 5-7).

As mentioned earlier, the carbon particles define pores some of whichare internal pores in the form of holes in the carbon particles; otherpores are gaps between adjacent carbon particles. Internal pores arealso referred to as micropores which generally have an equivalent radius(size) less than about 2 nanometers (nm) or 20 angstroms. External poresare also referred to as mesopores which generally have an equivalentradius (size) of over about 2 nanometers and up to about 20 nanometersor 200 angstroms. The total surface area present in a mass of carbonparticles is referred to as BET surface area, expressed in m² /gm. BETsurface area accounts for both mesopores and micropores present in themass. As used herein, the terms "pore" and "pores" refers to bothmesopores and micropores and also refers to both internal and externalpores unless otherwise indicated.

The carbon particles are sometimes referred to as grains or aggregates.

Preferably, the carbon particles 60 of the first and second groups eachhave pores with an average size in the range of between about 50 toabout 200 angstroms, and at least about 40% of the pores having a sizegreater than about 20 angstroms. Preferably, the first and second groupsof carbon particles 60 each has a BET surface area less than about 1,000square meters per gram.

Preferably, the first group of carbon particles 60 forming the anode hasa pH in the range of about 6 to about 7 and an average pore size in therange of about 90 to about 110 angstroms. Preferably, the first groupconsists of carbon particles obtained from Cabot Corp. and sold underthe name of "Vulcan XC-72R". The Vulcan XC-72R carbon particles areball-milled to enhance their properties for anode use.

Preferably, the second group of carbon particles 60 forming the cathodehas a pH in the range of about 8 to about 10 and the average pore sizeof the second group is in the range of about 60 to 80 angstroms.Preferably, the second group consists of carbon particles obtained fromNoury Chemical Corp., and sold under the name of "Ketjen black". TheKetjen black particles are used in an as-received condition to form thecathode.

As can readily be seen from the pH values, the first group is relativelymore acidic and less basic than the second group. Such pH values aremeasured in an aqueous suspension of a group of carbon particles.

Desirably, each one of the first and second groups of finely dividedcarbon particles has an equivalent average spherical diameter less thanabout 35 nm, and in a range of about 25 to about 35 nm.

Optionally, respective electrodes 40 (anode 42, cathode 44), furthercomprise respective first and second Teflonated (polytetrafluoroethylenecoated, impregnated) graphite sheets 80,82, adhered to a respectivesurface of membrane 46. The first group of catalyzed particles 60 isdisposed between the first surface 50 of membrane 46 and the first sheet80; the second group of catalyzed particles 60 is disposed between thesecond surface 52 and the second sheet 82. Each Teflonated sheet 80,82is about 71/2 to 13 mils thick.

Preparation of Electrolyte and Electrode Assemblies

In order to achieve localization of the platinum (Pt) catalyst 62 nearthe membrane-electrode boundary 50,52 and adjacent the membrane 46, toachieve optimal proton transport, the MEA 12 was prepared by applying aslurry of catalyzed carbon particles 60,62 and proton conductivematerial 64 onto the membrane 46. The applied slurry was thenhot-pressed to at least partially embed the particles 60,62 and protonconductive material 64 in the membrane 46. The embedded or insetparticles are at least partially set in surfaces of the membranealthough they may not be totally encompassed by the membrane or disposedbelow its surface.

It should be noted that the hot-pressed slurry forms electrodes 40essentially integral with the membrane 46. Although the catalyzed carbonparticles 60 may not totally penetrate the surfaces 50,52 of themembrane 46, the particles 60 do become embedded in surfaces 50,52 whenit softens during hot-pressing. Thus, some carbon particles 60 are atleast partially set in the membrane, although perhaps not entirelysubmerged below surfaces 50,52. Hot-pressing also achieves a peakdistribution of catalyst particles 62 adjacent surfaces 50,52 andintermediate region 53 of membrane 46. Due to the thickness ofintermediate region 53, contact between electrodes 42,44 is prevented.

The preferred perfluorocarbon sulfonic acid was used as the membrane 46and proton conductive material 64. Solution Technology supplies suchproton conductive material in dissolved form under the designation"NAFION SOLUTION". The slurry optionally containedpolytetrafluoroethylene (TEFLON). TEFLON is a trademark of Dupont.

The general method of making a combination membrane electrolyte andelectrode assembly (MEA) 12 includes:

a) forming a slurry of proton conductive material and at least one groupof finely divided carbon particles 60 and very finely divided catalyticparticles 62 supported on and in the carbon particles 60;

b) applying the slurry onto a first surface 50 of the membrane;

c) applying the slurry onto a second surface 52 of the membrane 46; and

d) heating while pressing the applied slurry for a time and at atemperature and compressive load sufficient to embed at least a portionof the particles 60,62 into the membrane 46 to thereby form the firstand second electrodes 40.

Optionally, the pressed membrane 46 and electrode 40 are treated toremove any organic and inorganic impurities, and if necessary, toconvert any non-hydrogen cation exchange sites of the proton conductivematerial to the hydrogen form.

Preferably, two groups of carbon particles are used to prepare first andsecond respective slurries applied to corresponding first and secondsurfaces of the membrane. The first group is characterized by greaterwater attraction and water retention properties.

In another embodiment, the slurry is applied to respective first andsecond Teflonated graphite sheets 80,82. Then, the wetted side of thefirst sheet 80 is applied to the first surface 50 of the membrane 46 andthe wetted side of the second sheet 82 is applied to the second surface52 of the membrane 46. The applied sheets 80,82 are hot-pressed to themembrane while being heated for a time and at a temperature andcompressive load sufficient to soften the membrane 46 and at leastpartially embed at least a portion of the particles 60 in the membraneto thereby form the first and second electrodes 40,42. The embedded orinset particles 60 are at least partially set in surfaces 50,52 of themembrane 46 although they may not be totally encompassed by the membrane46 or disposed below its surfaces 50,52.

Preferably, the compressive load is in the range of about 1,000 to about2,000 pounds per square inch (lb/in²) and the time and temperature arerespectively about 1 to about 5 minutes at about 120° C. to about 150°C.

EXAMPLE 1

In this example, a membrane electrode assembly (MEA) 12 was made byhot-pressing, using one group of carbon particles, Vulcan XC-72R in anas-received condition for both the anode and cathode. The properties ofVulcan XC-72R as received from the vendor are as per Table I.

The thin film MEA 12 was prepared from a slurry containing NAFION andcatalyzed carbon. The slurry was prepared by mixing catalyzed carbon, a5% NAFION solution (obtained from Solution Technology) and a Teflonsuspension (obtained from Dupont). The slurry was thinned to therequired extent using either 30 w/o (weight percent) t-butanol or water.The catalyzed carbon was 10 w/o platinum (Pt) catalyst dispersed onVulcan XC-72R carbon (C), either supplied by the Prototech Company orprepared using the Prototech process and carbons pre-treated byball-milling. The NAFION content was varied in the range of 20-50 w/oand the Teflon loading was varied in the range of 0-30 w/o.

The slurry was then coated on both sides of an untreated NAFION 117membrane of 175 microns thick by brushing. The coating was then dried at90°-95° C. for 30 minutes to remove the alcohol. Next, the appliedslurry was hot-pressed at either 120° C. for 5 minutes or at 150° C. for1 minute at a compressive load of 1000-2000 lb/in². This produced a thinfilm electrode at each of the two opposed, major surfaces of themembrane, and the electrodes were at least partially embedded into themembrane. The thin film MEA was then treated by boiling in 5% hydrogenperoxide (H₂ O₂) for 30 minutes, followed by boiling in 0.5M sulfuricacid (H₂ SO₄)) for 30 minutes, and finally rinsed several times inconductivity water. The membrane electrode assembly (MEA) had an overallthickness of about 200-225 microns (about 9 mils), consistent with thethin film construction of the electrodes at least partially embedded inthe membrane. The MEA exhibited only minor variation in membranethickness, demonstrating that good adherence is achieved without drasticdecrease in membrane thickness and without large variation in thethickness.

EXAMPLE 2

An MEA 12 was prepared by the method of Example 1, using Vulcan XC-72 asreceived, to form the anode 42 and cathode 44. No Teflon was used in theslurry and each electrode had a Pt loading of 0.08 mg/cm² (milligramsplatinum per square centimeter of electrode surface area). The MEA wastested by placing it in a graphite electrochemical cell. It produced 112mA/cm² at 0.5 V under ambient conditions of about 21° C. to 26° C. and 0to 4 psig.

EXAMPLE 3

An MEA was prepared by the method of Example 2 except that ball-milledVulcan XC-72 was used to form the anode 42. (See Table I for thespecifications of Vulcan as received and ball-milled). The MEA 12 withball-milled Vulcan (on the anode or fuel side only) produced 144 mA/cm²at 0.5 V under ambient conditions, described above.

The MEA 12 of Example 3 showed a 30% improvement compared to the MEA 12of Example 2. This is due to the effect of ball-milled Vulcan carbonparticles 60 as the substrate on the anode 42.

In the method of making the embedded electrodes in accordance with thepresent invention, it is typically necessary to treat the solid polymerelectrolyte membrane to remove organic and/or inorganic impurities usingH₂ O₂ and H₂ SO₄.

EXAMPLE 4

An alternate approach to preparing the thin film electrode assemblyinvolved applying the NAFION based slurry on a Teflonated graphite paper(supplied by Toray Industries, Japan), followed by hot-pressing to aproton exchange membrane (MEA-B). Unlike carbon cloth, the graphitepaper did not get soaked in the NAFION solution. The performance of thiselectrode-membrane assembly was compared in FIG. 8 with that of anassembly prepared by applying the slurry on the membrane as per theearlier examples (MEA-A). For MEA-A and MEA-B, each cathode had 15 w/oPt dispersed on Ketjen black and the anode had 5 w/o Pt on ball-milledVulcan XC-72R. The hydrogen-oxygen fuel cell performances for theassemblies (MEA-A and MEA-B) under ambient conditions were almostidentical, as shown in FIG. 8.

COMPARATIVE EXAMPLES

Another MEA assembly (MEA-C, FIG. 8) was prepared by applying the slurryonto the membrane and drying the slurry without hot-pressing. When thethin film electrode-membrane assembly was not hot-pressed, theperformance of the fuel cell decreased dramatically, as seen in FIG. 8(MEA-C), due to the high resistance of the assembly. Thus, thehot-pressing procedure is essential for obtaining good fuel cellperformance.

For further comparison, membrane electrode assemblies (MEA-X) wereprepared using commercially available Prototech electrodes with a Ptloading of 0.5 mg/cm². These conventional electrodes were thenimpregnated with a 5% NAFION solution by brushing and drying in a humidatmosphere. The NAFION loading was 1.0 mg/cm², which is consideredoptimum for these electrodes. Two electrodes were then placed on eithersides of a NAFION 117 membrane and the assembly was hot-pressed at 120°C. for 2 minutes, using a pressure of 2000 lb/in².

The Prototech based MEA-X was compared to an MEA prepared by the methodof Example 1 (MEA-A), as shown in FIG. 9. The catalyst activity,expressed in A/mg, is plotted against cell voltage in FIG. 9 for the twoassemblies, MEA-A and MEA-X. Thus, for example, at 0.4 V, the catalystactivity for the thin film assembly (MEA-A), is as high as 2.7 A/mg;whereas it is only 0.4 A/mg for the conventional assembly, thusdemonstrating a dramatic increase in the catalyst utilization in thethin film assembly by a factor of nearly 7. These results were based ona voltage scan rate: 5 mV/sec. Cell operated on hydrogen and oxygen at24° C. with a slight oxygen backpressure of 4 psig. We observed thathot-pressing a carbon cloth electrode causes a dramatic decrease andvariation in the membrane thickness. Thickness varied down to as low as3.5 mils compared to an unpressed thickness of 6.5 to 7 mils. Itappeared that the woven fibers of carbon cloth are being pressed to themembrane. In contrast, hot-pressed membranes of Examples 1-4 showed verylittle variation in thickness as slurry components were pressed in themembrane.

As a further demonstration of the effectiveness of hot-pressing, theadherence of the electrodes of MEA-A (prepared by hot-pressing) wascompared to the adherence of electrodes of MEA-C which was preparedwithout hot-pressing. The adherence of electrodes to each MEA was testedby lightly pressing typical adhesive (scotch-type) tape to an exteriorsurface of an MEA. That is, tape was applied to an exterior surface ofan electrode.

When the tape was removed from an electrode of the hot-pressed MEA ofthe invention (MEA-A), the tape was virtually clean. This was notsurprising given the shiny, smooth surface of the MEA of the invention,(MEA-A). This surface evidences the inextricable combining of NAFION,particles, and membrane which occur during hot-pressing. While notclearly defined, it appears some portions of membrane 46 adjacent theexterior surfaces 50,52 are softened and reformed with the appliedslurry as a recast or restructured portion of the membrane and MEA.

This does not occur without hot-pressing, thus the tape removed from thecomparative MEA (MEA-C), was covered with a black powder, evidencingeasy removal (poor adhesion) of slurry components from the comparativemembrane, (MEA-C).

Substrate Characteristics

Examples 2 and 3 show that the carbon substrate (particles) used todisperse the catalyst 62 exerted a profound influence on the retentionor rejection of water from the electrode. The ball-milled Vulcan XC-72Rcarbon particles 60 were beneficial to the hydrogen oxidation reaction.The hydrogen reaction and the water management of the membrane 46 wasassisted by the more hydrophilic carbon particles at the fuel electrode(anode 42), whereas the oxygen side (cathode 44) required morehydrophobic carbon particles 60. The ball-milled Vulcan carbon particles60 are more hydrophilic than the as-received or the heat treated Vulcancarbon particles 60.

The carbon particles 60 carrying catalyst 62 at the anode 42 must beless hydrophobic than the carbon particles 60 at the cathode 44, sincewater is removed from the anode 42 during fuel cell operation. Thus,carbon particles 60 of two different types prevent the drying of themembrane 46 and simultaneously support a high rate of mass transport ofhydrogen to the reaction sites. In the case of ball-milled Vulcanparticles, we found that ball-milling for about 2.5 hours reduces theaverage pore radius from 15.4 nm to 10.5 nm. (Table I). Ball-millingprobably breaks down the carbon particle aggregates, thereby enhancingthe area of contact of the carbon particles to oxygen. This probablyincreases the presence of oxygen groups on the surface, thus decreasingthe pH of the slurry from 6.8 to 6.2. A lower pH is an indication of aless hydrophobic (more hydrophilic) substrate, which is desirable forpromoting the water retention properties of the electrode/membraneinterface.

In order to further evaluate the effect of carbon substrate (particle)characteristics on cell performance, additional tests were conducted. Inthese tests, the carbon particles 60 for the hydrogen electrode (anode42) were held the same (ball-milled Vulcan XC-72R), and the carbonparticles 60 at the cathode 44 were varied to cover a spectrum ofsurface areas and pore distributions of various carbon particles. Thus,a group of carbon particles designated AX-21 carbon were chosen torepresent a high surface area carbon (2900 M² /g) with predominantlymicropores (98%), and a group of carbon particles designated Ketjenblack carbon (˜900 m² /g) was chosen for its high percentage ofmesopores (75%). Raven 5000 and Vulcan groups of carbon particles, whichhave a fair share of both micro and mesopores, were also investigated ascarbon substrate selections.

In the tests, the Pt loading was about 0.15 mg/cm² /cell. Theball-milled carbon for the fuel electrode (anode 42) was loaded with 5w/o Pt prepared by the sodium borohydride reduction method. The variousother carbon substrates for the cathode had 15 w/o Pt.

The hydrogen-oxygen fuel cell performance (shown in FIG. 5) underambient conditions increased in the following order: Raven5000<AX-21<Vulcan XC-72<Ketjen black. The performance of Ketjen blackwas 190 mA/cm² @0.5 V, compared to 130 mA/cm² @0.5 V for Vulcan XC-72R.The high performance of Ketjen black and the poor performance of AX-21may emphasize the important role of the dimensions of the electrode'scarbon particle pore dimensions in regulating the rejection of waterfrom the oxygen cathode.

Table I gives some properties of various carbon substrates (carbonparticle groups) along with the performance obtained for thehydrogen-oxygen fuel cell. Thus, Ketjen black has only a third of theBET area of AX-21, but the mesopore area (mesopores have diameters inthe ranges 3-20 nm and micropores have diameters less than 3 nm) is afactor of ten times higher.

                                      TABLE I                                     __________________________________________________________________________    Effect of Carbon Substrate                                                                                   Pore                                                            MESOPORE                                                                              Activity                                                                            Radius                                                  Total Area                                                                            AREA    at 0.4 V                                                                            (Average)                                      Substrate                                                                              (m.sup.2 /g)                                                                        pH                                                                              (m.sup.2 /g)                                                                      %   (mA/cm.sup.2)                                                                       (Å)                                        __________________________________________________________________________    Ketjen black                                                                           900   8.8                                                                             680 75  270   75                                             Vulcan XC-72                                                                           231.1 6.8                                                                             108   46.9                                                                            180   154.2                                          as received                                                                   Vulcan XC-72R                                                                          227.6 6.2                                                                             119   52.3                                                                            --    105.3                                          ball-milled 2.5 h                                                             Acetylene Black                                                                        64    6.6                                                                              60 95  170    51.0                                          AX-21    2900  4.8                                                                              68  2   84    9.9                                           Raven 5000                                                                             481   2.3                                                                             336 70   68    28.9                                          __________________________________________________________________________     Note:                                                                         Activity evaluated using 15 w/o Platinum prepared by the borohydride          method Pt loading: 0.15 (±0.02) mg/cm.sup.2 /cell.                         Meso = 3 to 20 nm or 30 to 200 Å. Å designates angstroms.        

While not wishing to be held to any particular theory, the following mayexplain the superior performance of ball-milled Vulcan XC-72R and Ketjenblack (as received) when used as the respective groups of carbonparticles (substrates) for the hydrogen anode and oxygen cathode,respectively. Pore size determines the water uptake properties of thecarbon particles. The smaller the average pore radius, the greater isthe tendency of the electrode to become flooded by the water produced bythe oxygen reduction reaction. This is apparently due to capillaryforces. This flooding will eventually shut off the supply of oxygen tothe reaction sites. The mesopore area alone cannot explain the trend inthe observed results. For example, Raven 5000 has nearly half themesopore area of Ketjen black, but the performance is even worse thanAX-21. Thus, the chemical properties of the carbon particles have animportant role to play in determining the hydrophobicity of the carbonsubstrate.

An important chemical property of carbon particles is the pH of itsaqueous carbon slurry. FIG. 6 shows a plot of hydrogen-oxygen fuel cellperformance (at ambient conditions and Pt loading of 0.15 mg/cm² /cell)against the pH of the aqueous carbon slurry. The performance is seen toincrease with an increase in the pH. The pH values for various carbonparticles are given in Table I. Raven 5000, which shows the lowestperformance, also shows the lowest pH (at about 2). It appears that theorigin of the pH effect lies in the nature of the surface functionalgroups. The reactions that determine the pH of the aqueous slurry areshown by reference to examples below. Ketjen black>C+2H20=>C--H30++OH--. Raven 5000>--COOH+H20=>--COO-- +H30+.

Acid pH values are caused by the presence of carboxylic (--COOH) groupson the carbon surface. When oxygen groups are strongly bonded to thesurface, the substrate becomes very hydrophilic. On the other hand, forcarbon particles such as Ketjen black, which is free from oxygen groupsand presents a bare carbon surface, the aqueous equilibrium is set up inthe alkaline pH range (7-9). A bare carbon surface (or a hydrogenated Csurface) is likely to be more hydrophobic.

Having identified the carbon slurry pH and the mesopore area asproperties affecting performance, a multiple regression plot of theperformance against the two variables is shown in FIG. 7. Actual currentdensity is correlated to calculated current density by the equation:i=a+b pH+c A m

The data agree with the regression line for five different groups ofcarbon particles possessing a wide spectrum of properties. Apparently,Ketjen black carbon particles provide the optimum carbon substrate foroxygen reduction under ambient conditions, because its hydrophobicity isconditioned by a high surface pH (about 8.8) and a high percentage ofmesopores.

Using Vulcan carbon particles 60 at the anode 42 and Ketjen black carbonparticles 60 at the cathode 44, we investigated the effect of usingdifferent catalyst loadings on the hydrogen and the air (oxygen) sidesof the thin film electrode-membrane assembly (MEA) 12. Slurries wereprepared as per Example 1 but without Teflon. The electrode thicknesswas held constant at about 25 microns. The Pt loading was varied byusing 5 w/o Pt/C (weight percent platinum to carbon) for the hydrogenside (anode 42) and 10, 15, 20, and 30 w/o Pt/C for the oxygen side(cathode 44). The total Pt loadings and performance at two voltagelevels are shown in Table II. It is clear that reducing the Pt loadingto 0.03 mg/cm² (milligrams per square centimeter of electrode surfacearea) on the hydrogen side (anode 42) still leads to high cellperformances as shown in the Table II. The optimum catalyst utilizationon the oxygen side (cathode 44) occurs at a Pt loading of 0.06 mg/cm²using 10 w/o Pt on Ketjen black carbon particles.

The values of cell performance presented in Table II were derived asfollows. The 0.095 mg/cm² /cell Pt loading is based upon the use of 1.63mg Pt per 25 cm² (0.065 mg/cm²) of cathode electrode area, and 0.75 mgPt per 25 cm² (0.03 mg/cm²) of anode electrode area. Thus, loading percell is (1.63 mg+0.75 mg)/25 cm or 0.095 mg/cm² /cell. This is based onthe fact that the area spanned by each electrode is essentiallyequivalent and the electrodes are aligned opposite one another on eachside of the membrane. Thus, in the expression "mg/cm² per cell", thenumerator is total Pt weight and the denominator is area of eitherelectrode, as described above.

                  TABLE II                                                        ______________________________________                                                      CELL PERFORMANCE                                                Pt Loading                                                                              Pt w/o    mA/cm.sup.2                                                                             mA/cm.sup.2                                                                          mA/mg                                    (mg/cm.sup.2 /cell)                                                                     (Cathode) @ 0.6 V   @ 0.5 V                                                                              @ 0.6 V                                  ______________________________________                                        0.095     10        160       238    1684                                     0.122     15        190       280    1557                                     0.173     20        191       277    1104                                     0.233     30        202       --      867                                     ______________________________________                                         Anode Pt loading was held constant at 0.03 mg/cm.sup.2 using 5 w/o Pt.        Cathode Pt loading was varied as shown, and ranges from 0.065 mg/cm.sup.2     at 10 w/o to 0.203 mg/cm.sup.2 at 30 w/o.                                

In a preferred embodiment ball-milled Vulcan XC-72 and Ketjen blackcarbon particles provide the best substrates for the hydrogen oxidation(anode 42) and the oxygen reduction (cathode 44) reactions respectivelyin the fuel cell operating under ambient conditions. Pt loadings as lowas 0.03 mg/cm² for the fuel anode 42 and 0.06 mg/cm² for the oxygencathode 44 have been found to be sufficient to yield high performancesunder ambient conditions (160 mA/cm² @0.6 V and 238 mA/cm² @0.5 V) usingthe optimum carbon substrates.

The MEA's of the invention have been compared to MEA's formed withouthot-pressing. Our hot-pressed MEA is relatively thinner and has embeddedelectrodes, due to pressing. This reduces ohmic drop and increasesperformance.

Importantly, the thin film technique affects the dispersion of Pt in theMEA. We observed a peak distribution of Pt near the membrane-electrodeboundary, that is, adjacent surfaces 50,52 and intermediate region 53 ofmembrane 46. The Pt is thus localized to provide maximum catalyticactivity. Conventional MEA's, prepared from Prototech electrodes, arenot known to provide this beneficial localization of Pt. Other keyadvantages of the method are: the effective dispersion of the protonconductive material, effective water management by the two groups ofcarbon particles and low catalyst loadings.

The invention may be practiced using other proton conductive electrolytemembranes, slurry material, and catalyst. The temperature ofhot-pressing is mainly dependent on the thermal and mechanical integrityof the electrolyte membrane. Thus, higher or lower temperatures andpressures may be used for hot-pressing.

While this invention has been described in terms of certain embodimentsthereof, it is not intended that it be limited to the above description,but rather only to the extent set forth in the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined in the appended claims.

We claim:
 1. A combination electrolyte and electrode structure for anelectrochemical cell comprising:a) a solid proton conductive polymerelectrolyte membrane having first and second spaced apart opposedsurfaces and an intermediate region; b) first and second electrodes eachadhered to a respective one of the first and second surfaces of themembrane; c) the first and second electrodes respectively comprisingfirst and second groups of carbon particles, each one of the carbonparticle groups including a plurality of the carbon particles havinginternal and external surfaces defining a plethora of pores within andbetween the carbon particles, very finely divided catalytic particlessupported on the internal and the external surfaces of the carbonparticles, and a proton conductive material intermingled with the carbonparticles and the catalytic particles; d) the first group of the carbonparticles forming the first electrode being more hydrophilic than thesecond group of carbon particles forming the second electrode; and e) aportion of the carbon particles forming a respective one of theelectrodes being at least partially embeded in the membrane.
 2. Thecombination electrolyte and electrode structures according to claim 1,wherein the first and second carbon particle groups are eachcharacterized by a degree of acidity or basicity, and wherein the firstgroup is more acidic than the second group.
 3. The combinationelectrolyte and electrode structure according to claim 1, wherein eachone of the first and second carbon particle groups is characterized by apH, and wherein the first group has a pH less than the pH of the secondgroup.
 4. The combination electrolyte and electrode structure accordingto claim 1, wherein each one of the carbon particle groups has poreswith an average size in the range of between about 50 to about 200angstroms, and at least about 40% of the pores having a size greaterthan about 20 angstroms.
 5. The combination electrolyte and electrodestructure according to claim 3, wherein the first carbon particle grouphas a pH in the range of about 6 to about 7 and the second group has apH in the range of about 8 to about
 10. 6. The combination electrolyteand electrode structure according to claim 4, wherein the first carbonparticle group has an average pore size in the range of about 90 toabout 110 angstroms and the average pore size of the second group is inthe range of about 60 to about 80 angstroms.
 7. The combinationelectrolyte and electrode structure according to claim 1, wherein thecatalytic particles of the first and second electrodes are each ofplatinum, and wherein the platinum loading of the second electrode isless than about 0.07 milligrams per cm² of electrode surface area, andthe platinum loading of the first electrode is less than that of thesecond electrode.
 8. A combination according to claim 1, furthercomprising first and second sheets of graphite paper impregnated withpolytetrafluoroethylene, the first carbon particle group disposedbetween the first surface of the membrane and the first sheet of saidgraphite paper and the second carbon particle group disposed between thesecond surface of the membrane and the second sheet of said graphitepaper.
 9. The combination electrolyte and electrode structure accordingto claim 1, wherein the catalytic particles are platinum supported onthe carbon particles in a proportion, based on 100 parts by weight,where the platinum particles constitute up to about 15 parts and thecarbon particles the balance.
 10. The combination electrolyte andelectrode structure according to claim 1, wherein the proton conductivematerial consists essentially of a perfluorocarbon sulfonic acidpolymer.
 11. The combination electrolyte and electrode structureaccording to claim 1, wherein the proton conductive material consistsessentially of a mixture of perfluorocarbon sulfonic acid polymer andpolytetrafluoroethylene.
 12. The combination electrolyte and electrodestructure according to claim 1, wherein the electrolyte membrane and theproton conductive material each comprise perfluorocarbon sulfonic acidpolymer.
 13. The combination electrolyte and electrode structureaccording to claim 1, wherein each one of the electrodes is formed ofthe proton conductive material and the catalytic and carbon particles ina proportion, based on 100 parts, of about 30 to about 50 parts protonconductive material and the balance the catalytic and carbon particles.